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RHT & SE Flashcards

1
Q

What is RHT?

A

Radiative heat transfer

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2
Q

What is SE?

A

Solar energy

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3
Q

What causes RHT?

A

Energy transitions within molecules

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4
Q

Does RHT require matter?

A

To emit and absorb but not to transfer

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5
Q

RHT varies with..?

A

Temperature, wavelength and direction

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6
Q

What are spectral properties?

A

For surfaces, different materials => different spectral reflectance, transmittance, and absorptance

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7
Q

What is important to think about with big temp. differences when it comes to RHT?

A

Heat transfer between surfaces – in both directions!

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8
Q

How to calculate a surfaces emission?

A

Emissivity (πœ€_𝑖) , blackbody radiation (𝐸_𝑏𝑖 =πœŽπ‘‡_𝑖^4)
𝐸_𝑖 =πœ€_𝑖 * 𝐸_𝑏𝑖

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9
Q

How to calculate a surface’s emission?

A

Emissivity (πœ€_𝑖) , blackbody radiation (𝐸_𝑏𝑖 =πœŽπ‘‡_𝑖^4)
𝐸_𝑖 =πœ€_𝑖 * 𝐸_𝑏𝑖

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10
Q

Emits =

A

UtstrΓ₯lar (ut frΓ₯n ytan)

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11
Q

Irradiates =

A

BestrΓ₯las (in mot ytan)

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12
Q

Irradiation betecknas

A

G_i

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13
Q

Irradiation delas upp i tre delar, vilka? Och hur betecknas de?

A
  • Reflected: 𝜌_𝑖*𝐺_𝑖
  • Absorbed: 𝛼_i*𝐺_𝑖
  • Transmitted: 𝜏_i*𝐺_𝑖
  • 𝜌_𝑖 +𝛼_𝑖 +𝜏_𝑖 =1
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14
Q

What is the total outgoing radiation from a surface called (and noted)?

A

Radiosity J_i,
𝐽_𝑖 =𝐸_𝑖 +𝜌_𝑖*𝐺_𝑖

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15
Q

Blackbody - three β€œassumptions”

A
  1. No surface can emit more energy than a blackbody (πœ€ =1) (given a temp and wavelength)
  2. The emitted radiation is independent of direction (diffuse emitter) - alltsΓ₯ samma utstrΓ₯lning Γ₯t alla hΓ₯ll
  3. All irradiation is absorbed by a blackbody (𝛼 =1)
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16
Q

RHT - intensity?

A

Radiation in one direction (small space angle), independent of distance (if no absorption along the way) 𝑑Ω1 =𝑑Ω2

17
Q

RHT - heat flux?

A

Radiation from/to all directions (hemisphere), proportional to 1/L^2 (if no absorption on the way) 𝑑A1 ≠𝑑A2

18
Q

What does opaque surface mean?

A

no radiation is transmitted through the surface
* 𝜏_𝑖=0
* 𝛼_𝑖 +𝜌_𝑖 =1

19
Q

Diffuse surface/irradiation meaning?

A
  • Surface – the emitted radiation is equally large in all directions
  • Irradiation – the incoming radiation is equally large from all directions
20
Q

What does grey body mean?

A

The emissivity is independent of wavelength
* πœ€πœ† =πœ€
* 𝛼 =πœ€

21
Q

The sun acts as a..?

22
Q

View factors, meaning?

A

Radiation, hur ytor som ser varandra strΓ₯lar pΓ₯ varandra

23
Q

View factors, rules:

A
  1. Reciprocity relation - 𝐹_12𝐴_1 =𝐹_21𝐴_2
  2. Summation rule, F_ij = 1, med j = [1,N] (enclosure of N surfaces) (om ej enclosure lΓ€gre Γ€n 1, aldrig ΓΆver)
24
Q

𝐹_𝑖𝑗 betecknar vad?

A

The portion of the total radiation from surface 𝑖 that reaches surface 𝑗

25
Om det finns N st ytor, hur mΓ₯nga view factors finns?
N^2
26
SE: irradiation fom the sun, 3 things to think about
1. Irradiation from the sun close to blackbody radiation 2. Gases in the atmosphere absorb some wavelengths (solar radiation to earth, emitted radiation from earth. polar gases, H2O, CO2, NH3, CH4) 3. The irradiation angle affects how much radiation that reaches the surface
27
Name some ways we can make use of solar radiation.
Solar thermal collectors, solar cells, solar power
28
What do solar thermal collectors generate?
Generates heat. Heats water.
29
What do solar cells generate?
PV, photovoltaic - omvandling ljus => elektricitet mha halvledande material Generates electricity (small scale)
30
What does solar power generate?
CSP - concentrated solar power Generates electricity, large scale (med turbin, iaf i vΓ₯rt ex.)
31
Solar thermal collectors and cells: Properties for cover glass:
High transmissivity for short wavelengths Low transmissivity for long wavelengths
32
Solar thermal collectors and cells: Properties for tubes:
High absorptivity for short wavelengths Low emissivity for long wavelengths (high absorptivity => high emissivity, 𝛼 =πœ€)
33
Explain solar cells
* Two layers of semiconductor material with opposite charges * Total efficiency is about 20% for most common cells
34
Why is a concentrator used in a CSP?
Increase irradiation to a reciever
35
CSP stands for?
Concentrated solar power
36
Concentration factor C =
A_mirrors/A_reciever
37
Name a few CSP mirror challanges
Shadowing, blocking (requires larger space in that case Not always direct sunlight - scattered sunlight and diffuse irradiation, harder to redirect to reciever
38
Calculate emitted power from surface
Q = E * A [W/m^2 * m^2 = W]

SEE020 (4 decks)

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